Evidence is provided to show that chorionic gonadotrophins extracted from the human, rhesus monkey and marmoset placentae have antigenic determinants in common. Similar slopes were obtained for these gonadotrophins in a radioimmunoassay for human chorionic gonadotrophin (HCG). The biological activity of the monkey gonadotrophins was neutralized by anti-HCG serum. When the gonadotrophic activity of the monkey placental extracts was assayed biologically and immunologically, using HCG as a standard, similar results were obtained. Higher values were obtained by the immunoassay than by the bioassay when extracts of human placenta were assayed using the same HCG standard.